The field of the invention relates to a control of a vehicle equipped with an automatic or manual transmission, and in particular to an engine torque control.
Vehicles have an internal combustion engine coupled to a torque converter coupled to an automatic transmission. They can also have various accessories such as an air conditioning (A/C) compressor, power steering pump, or alternator intermittently coupled to the engine. Having an accurate model of engine torque contributes to better engine control, improved vehicle performance, and improved driver comfort during accessory engagement or transmission gear shifts. For example, decreasing engine torque during transmission gear downshifts reduces shift shock and increases driver satisfaction.
One method of estimating engine torque is described in U.S. Pat. No. 5,910,176. This method comprises first calculating torque converter input torque as a function of operating conditions, then calculating torque converter input torque as a function of torque converter output speed. The two values are then compared and used to calibrate a computer based model of engine torque. Engine torque can then be determined as a function of operating conditions based on the calibrated computer model. The computer model accounts for torque losses due to friction and accessory loading. These losses are determined using a map of power loss versus engine speed. Another method is described in U.S. Pat. No. 5,826,208. Engine output torque is calculated by using engine characteristics, and torque converter torque is calculated using torque converter characteristic. The two methods are selectively used depending on the magnitude of the torque converter slip ratio. When the slip ratio is lower than a small preselected value, the method using the torque converter characteristic is used. When the slip ratio is higher than a small preselected value, the method using the engine characteristic is used.
The inventors herein have recognized a disadvantage with these approaches. In particular, when the torque converter torque is interpolated from the characteristic map of the torque converter ratio as related to the speed ratio, variations in the torque converter fluid characteristics can cause reduced accuracy. In other words, estimating torque using torque converter characteristics is susceptible to variations in hydraulic fluid properties. For example, when the fluid is cold, it is more viscous, and torque prediction will not be as accurate. Thus, torque estimates are affected by the variation in hydraulic fluid properties.
Another disadvantage is that changes in friction work and pumping work losses are not considered since predetermined functions are used. Also, neither of the above methods consider a locked torque converter, which is a large proportion of vehicle drive cycles.
An object of the present invention is to provide a method for calculating load torque of an accessory device coupled to an engine.
The above object is achieved and disadvantages of prior approaches overcome by a method for use with an internal combustion engine in a powertrain coupled to a device having an input speed and an output speed, the method comprising indicating that powertrain output torque is less than a predetermined value based on the input speed and the output speed of the device; and learning a powertrain torque adjustment value in response to said indication.
An advantage of the above aspect of the invention is that improved estimate of engine torque can be calculated, and thus a more precise method of engine control is achieved. Another advantage is that improved drive feel is achieved during accessory engagements.
Other objects, features and advantages of the present invention will be readily appreciated by the reader of this specification.